In the manufacture of polyene compounds containing a 3,7-dimethyl-nona-1,3,5,7-tetraen-1-yl group by reacting triarylphosphonium salts with aldehydes (e.g., as used in the manufacture of vitamin A acetate by reacting a .beta.-ionyliden-ethyltriphenylphosphonium halide with .gamma.-acetoxytiglic aldehyde), the reaction has hitherto been carried out in a homogeneous phase using, for example, dimethylformamide, acetonitrile, acetone, dioxane or isopropanol as the solvent. Various disadvantages are encountered when the process is carried out in this manner. In particular, in order to obtain yields which are reasonably satisfactory, the reaction must be carried out at a very low temperature (i.e., at a temperature of below +5.degree. C. to about -30.degree. C.), which requires a substantial cooling capacity. If the reaction is carried out at a temperature greater than +5.degree. C., the yield of end product (e.g., of vitamin A acetate) is considerably reduced (e.g., to about 80%). Furthermore, the proportion of cis-isomers in the end porduct is relatively high.